The invention relates to an optical receiver comprising a photodiode connected to a biasing circuit. The photodiode is further connected to an output of the receiver via a series arrangement of a preamplifier, a control amplifier and an output amplifier whose output is connected via a detector to one input of a differential amplifier which has another input connected to a source of reference voltage and which also has an output connected to a control input of the control amplifier.
In such optical receivers the possibility of measuring the optical input level during operation is very much desired. This makes it possible to measure the attenuation of the connection (optical fiber cable welded joints and optical plugs) during the initial test procedure of an optical communication system. This is of great advantage as otherwise, when testing the optical input signal, the optical connection between the cable and the optical receiver must be interrupted. In addition, it is possible to carry out maintenance tests during operation to measure any change in the optical input level and, acting thus, to have the possibility of taking corrective measures at an early stage.
In this case there is the additional possibility of detecting any aging of the glass fiber cable, and, as the case may be, of connecting an alarm thereto.